The present invention relates to an electrically driven linear actuator in accordance with the preamble of claim 1.
Electrically driven linear actuators of the generic type are used for reciprocally moving a particular object (mass) in a linear direction, where it is generally possible, e.g. in comparison with hydraulically driven linear actuators, to achieve a substantially better accuracy of actuation, while control is less complicated. Furthermore there is no risk of soiling by leaked hydraulic oil.
Previously known electrically driven linear actuators (cf., e.g., U.S. Pat. No. 6,145,395 or U.S. Pat. No. 4,712,441) act on the object to be actuated through a tubular actuator element which is frictionally coupled to this object in a suitable manner on the output side. The tubular actuator element is mounted so as to slide in an axial direction inside a main pipe (which generally also serves as a housing) and on the input side is coupled to a spindle nut which, in turn, is inserted on a spindle rotatably mounted inside the main pipe and made to rotate by an electric motor (as a general rule through the intermediary of a reducing gear). Under the condition that the spindle nut is held non-rotatably (either by a stop inside the main pipe or by the object to be moved itself), each rotation of the spindle thus causes an axial displacement of the spindle nut, so that the actuator element coupled to the latter is telescopingly displaced relative to the main pipe. As, on the other hand, the main pipe is fixedly mounted relative to the object to be moved, either a tensile force or a thrust force is exerted on this object in accordance with the motor's direction of rotation, whereby the object is caused to undergo a corresponding linear displacement.
U.S. Pat. No. 4,500,805 discloses an electrically driven linear actuator wherein the actuator acting on the object to be moved consists of altogether four rod-shaped actuator elements concentrically arranged, at a uniform angular distance of 90 degrees, about the centrally extending spindle. By this fourfold arrangement it is intended to improve rigidity of the linear actuator, which is of significance particularly in cases of very long actuators. The fundamental function of this actuator does, however, in other respects not differ from the actuators having only one actuator element as described at the outset.
One application of the linear actuators of the invention is, e.g., a driving or flight simulator where the simulation object has to be moved in altogether six degrees of freedom; accordingly, six separately controllable linear actuators are required for this purpose.
A fundamental problem of the electrical linear actuators of the invention resides in the undesirable generation of noise and particularly of vibrations; both types of emissions are a decisive drawback in many applications, however particularly so in the mentioned driving or flight simulation technology, for the measurement values to be obtained may thus be falsified in an inadmissible manner. For instance, in such simulation processes the quality of simulation depends in a high degree on a largely accurate and unfalsified reproduction, not only of visual information and force feedback, but also of the generated acceleration values and simulated sounds (audio information about the simulated events); the two operation parameters mentioned last are, however, falsified by the vibration inherent in the known linear actuators and the noises generated by them, respectively.
Investigations have shown that the main cause for the generated vibrations is to be found in the mechanical transmission of the motor's rotation to the spindle (and thus inside the transmission), in the transformation of the rotational movement into a linear displacement owing to the spindle meshing with the spindle nut, and in the drive-side mounting of the spindle. The noise generation, on the other hand, is particularly due to those oscillations of the object to be moved that are stimulated by the vibrations of the linear actuator mechanically coupled to the object.
As it is not possible to achieve a significant reduction of vibrations even by using very high-quality transmissions, spindles and/or bearings, it was finally contemplated to couple the object to be moved to the end of the actuator by means of an intermediately arranged elastic member. Apart from the fact that this tends to increase the length of the linear actuator in an inadmissible degree, the following drawbacks of this solution were moreover stated:
1) Even after comparatively short operation, an elastic intermediate member shows fatigue phenomena which falsify the desired target displacement distance.
2) It is difficult to adjust the attenuation characteristics of the intermediate member for any operating conditions so as not to attenuate oscillating movements to be generated of the object but still allow to neutralize the vibration oscillations (which are, as a rule, of higher frequencies).3) The elastic intermediate member is exposed to the respective environmental conditions, and it is often difficult to find a material having a sufficient temperature strength in the range of −21 to 70° C. and at the same time a sufficient resistivity against oil or solvents.4) The intermediate member is deformed, not only elastically in the actuation direction, but also in the transverse directions relative thereto; while the deformation in the actuation direction may be compensated by corresponding actuating oder control measures in the control of the spindle drive, the deformations in the other directions may be compensated only with difficulty and therefore falsify the results of a simulation.